Steerable paravane system for towed seismic streamer arrays

ABSTRACT

A paravane for a seismic acquisition system includes a float, a frame suspended from the float, deflectors affixed to the frame, and means for coupling a tow rope to a lead-in functionally extending between a forward end and an aft end of the frame. The paravane includes means for selectively changing an effective position along the lead-in of the means for coupling the tow cable.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to the field of marine seismic surveysystems. More particularly, the invention relates to paravanes used toexert controllable lateral force on a seismic streamer array.

2. Background Art

Marine seismic survey systems are used to acquire seismic data fromEarth formations below the bottom of a body of water, such as a lake orthe ocean. Marine seismic survey systems known in the art typicallyinclude a seismic vessel having onboard navigation, seismic energysource control, and data recording equipment. The seismic vessel istypically configured to tow one or more seismic sensor “streamers” (longcables with sensors at spaced apart locations along the cable) throughthe water. At selected times, the seismic energy source controlequipment causes one or more seismic energy sources, which may be towedin the water by the seismic vessel or by another vessel, to actuate.Signals produced by the sensors in the one or more streamers ultimatelyin response to energy from the seismic source are conducted byelectrical, radio and/or optical telemetry to the recording equipment,where a record indexed with respect to source actuation time is made ofthe signals produced by each sensor (or groups of such sensors). Therecorded signals are later interpreted to infer the structure and/orcomposition of the Earth formations below the bottom of the body ofwater.

As explained above, the one or more streamers are in the most generalsense long cables that have seismic sensors disposed at spaced apartpositions along the length of the cable. A typical streamer can extendbehind the seismic vessel for several kilometers. More recently, marineseismic acquisition systems have been designed that include a number ofsuch streamers towed by the seismic vessel parallel to each other. Atypical multiple streamer system includes a plurality of “lead-incables” each coupled to a forward end of one of the streamers. Thelead-in cables are used to deploy the streamers from the seismic vesseland to maintain the streamers at a selected distance behind the vessel.

The streamers are fixed near their forward ends to a spreader cable or“super wide” cable. The spreader cable extends in the water transverselyto the direction of motion of the vessel, and when maintained in thecorrect tension, substantially fixes the relative lateral positions ofthe forward ends of the streamers. The spreader cable is maintained intension by a device coupled to each end of the spreader cable called aparavane.

The paravanes include diverters or similar suitably-shaped deflectingplates that redirect the motion of the water past the paravane laterallyto produce some amount of lateral force ultimately applied to thespreader cable. The lateral force exerted by the paravanes is related tothe shape and orientation of the deflecting plates and the speed ofmotion of the paravanes through the water. One issue of concern toseismic survey system operators using paravanes known in the art is thatthe lateral force exerted by the outermost paravane when the vesselturns can be excessive at ordinary towing speeds because the outermostparavane will be moving considerably faster than the vessel, dependingon the lateral distance between such paravane and the centerline of thevessel. Thus, using paravanes known in the art, it is frequentlynecessary to limit the vessel speed during turns, thus reducing theefficiency with which a survey can be performed. Where there are watercurrents that move in a direction along the direction of motion of theseismic vessel, the amount of lateral force exerted by paravanes knownin the art will increase with current flow opposite the vessel directionbecause of the increased water velocity past the paravane, or willdecrease with water current flow in the same direction as the seismicvessel motion because of decreased water velocity past the paravane.Where there are water currents moving transversely to the direction ofmotion of the seismic vessel, the seismic streamer array may be movedlaterally in a manner that is difficult for the system operator tocompensate or control.

It is also known in the art that the most suitable configuration for thearray of cables that ultimately couples the paravanes to the spreadercable (called a “bridle”) and/or tow rope may vary depending on theparticular paravane used, and on actual vessel motion conditions. In theevent the bridle actually used in any survey is not optimal for theexisting equipment configuration and survey conditions, it is frequentlynecessary to retrieve the paravane and reconfigure the bridle. Suchretrieval and reconfiguration operations can be costly and timeconsuming.

There continues to be a need for improved structures for paravanes andbridles to increase seismic survey efficiency.

SUMMARY OF THE INVENTION

A paravane for a seismic acquisition system according to one aspect ofthe invention includes a float, a frame suspended from the float,deflectors affixed to the frame, and means for coupling a tow rope to alead-in functionally extending between a forward end and an aft end ofthe frame. The paravane includes means for selectively changing aneffective position along the lead-in of the means for coupling the towcable.

Another aspect of the invention is a paravane for a seismic acquisitionsystem. A paravane according to this aspect of the invention includes afloat, a frame suspended from the float, deflectors affixed to theframe, and a bridle coupled to the frame at selected positions. Thebridle includes at least one cable coupled at one end proximate aforward end of the frame and at least one cable coupled at one endproximate an aft end of the frame. Means coupled to the other end ofeach of the forward and aft cables are provided for selectively changingan effective tow point between the other ends of the forward and aftcables.

A marine seismic survey system according to another aspect of theinvention includes a seismic vessel. A plurality of seismic sensorstreamers are towed by the vessel at laterally spaced apart positions. Aspreader cable extends substantially transversely to a direction ofmotion of the seismic vessel. Each of the streamers coupled at itsforward end to the spreader cable. A paravane is coupled to each end ofthe spreader cable. Each paravane includes a frame suspended from thefloat, at least one deflector affixed to the frame and a bridle coupledto the frame at selected positions. The bridle includes at least onecable coupled at one end proximate a forward end of the frame and at theother end proximate an aft end of the frame. The paravane includes meansfunctionally associated with the cable for selectively changing aneffective tow point between the ends of the cable. A tow rope is coupledto the tow point of each bridle at one end and at the other end thereofto the seismic vessel.

A method for controlling a lateral force exerted by a paravane towed bya vessel includes moving the vessel through a body of water, couplingmotion of the vessel to an effective tow point associated with a firstparavane and changing the effective tow point of the first paravane toprovide a selected angle of attack thereof.

One embodiment of controlling the effective tow point includes thefollowing. Motion of the vessel is coupled through a tow rope to acoupling point on a bridle. The bridle includes at least one cablecoupled at a first end to the coupling point and at a second endproximate a forward end of the paravane. The bridle includes at leastone cable coupled at a first end to the coupling point and at a secondend proximate an aft end of the paravane. The method includesselectively changing a distance between the coupling point and the firstend of each of the forward coupled and aft coupled cables, therebychanging the angle of attack of the paravane in the water.

Other aspects and advantages of the invention will be apparent from thefollowing description and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a plan view of a typical marine seismic survey system thatcan include steerable paravanes according to the invention.

FIG. 2 shows one embodiment of a paravane.

FIG. 3 shows an adjustable position bridle that can be used with theparavane shown in FIG. 2.

FIG. 4 shows one example of a node position controller coupled to abridle and a tow rope.

FIG. 5 shows an arrangement of a bridle coupled to a paravane diverterframe using a node position controller.

FIGS. 6A and 6B show one example of a bridle node.

FIG. 7 shows a motor/gear combination that may be used with variousexamples of a node position controller.

FIGS. 8A, 8B and 8C show various components of a planetary reductiongear used to convert motion of the motor of FIG. 7 to driving force torotate the sprocket shown in FIG. 4.

FIG. 8D shows the sprocket coupled to the planetary gear of FIGS. 8Athrough 8C.

FIG. 9 shows an oblique view of the node position controller.

FIG. 10 shows example circuitry for a direction controller unit.

DETAILED DESCRIPTION

FIG. 1 shows a typical marine seismic survey system that can includeparavanes and paravane connecting bridles according to the variousaspects of the present invention. The acquisition system includes aseismic vessel 10 that moves along the surface of a body of water 11such as a lake or the ocean. The seismic vessel 10 may include thereoncertain electronic equipment, shown at 12 and for conveniencecollectively called a “recording system.” The recording system 12typically includes a recording unit for making a record with respect totime of signals generated by various seismic sensors in the acquisitionsystem. The recording system 12 also typically includes navigationequipment to determine at any time the position of the vessel 10 andeach of a plurality of seismic sensors 22 disposed at spaced apartlocations on streamers 20 towed by the vessel 10. The foregoing elementsof the recording system 12 are familiar to those skilled in the art andare not shown in the figures for clarity of the illustration. In thepresent invention, the recording unit 12 may also include devices forcontrolling operation of a paravane steering device, as will be furtherexplained below.

The seismic sensors 22 can be any type of seismic sensor known in theart such as motion responsive sensors, acceleration sensors, pressuresensors, pressure time gradient sensors or any combination thereof. Theseismic sensors 22 measure seismic energy primarily reflected fromvarious structures in the Earth's subsurface below the bottom of thewater 11. The seismic energy originates from a seismic energy source(not shown) deployed in the water 11. The recording system 12 may alsoinclude seismic energy source control equipment (not shown separately).One or more seismic energy sources (not shown in the figures forclarity) may be towed by the seismic vessel 10 or by another vessel (notshown) nearby.

In the seismic data acquisition system shown in FIG. 1, there are sixseismic sensor streamers 20 towed by the seismic vessel 10. The numberof seismic sensor streamers may be different in any particularimplementation of an acquisition system according to the various aspectsof the invention, therefore, the number of streamers shown in FIG. 1 isnot intended to limit the scope of the invention. As explained in theBackground section herein, in seismic acquisition systems such as shownin FIG. 1 that include a plurality of laterally spaced apart streamers,the streamers 20 are coupled to towing equipment that maintains thestreamers 20 at selected lateral positions with respect to each otherand with respect to the seismic vessel 10. As shown in FIG. 1, thetowing equipment can include two paravane tow ropes 16 each coupled tothe vessel 10 at one end through a winch 19 or similar spooling devicethat enables changing the deployed length of each paravane tow rope 16.The other end of each paravane tow rope 16 is functionally coupled to aparavane 14, typically through a set of cables called a “bridle”, whichwill be further explained herein. The paravanes 14 are each configuredto provide a lateral component of motion to the various towingcomponents deployed in the water 11 when the paravanes 14 are towed inthe water 11, and as will be explained below, in various aspects of thepresent invention such lateral component of motion can be adjustable orcontrollable. Lateral in the present description means essentiallytransverse to the direction of motion of the vessel 10. The lateralmotion component of each paravane 14 is opposed to that of the otherparavane 14, and is generally in a direction outward from the centerlineof the vessel 10. The combined lateral motion of the paravanes 14separates the paravanes 14 from each other until they put one or morespreader ropes or cables 24, functionally coupled end to end between theparavanes 14, into tension.

As used in the present description, the term “cable” generally means adevice that includes one or more electrical and/or optical conductorstherein for carrying electrical power and/or signals from the vessel 10to and/or from various components of the seismic acquisition system. Acable as used in the present context may also include various forms ofarmor or other device to carry axial loading along the cable, and thusmay be a load bearing part of the acquisition system. The term “rope” isgenerally used in the present description to refer to a flexible, axialload carrying member that does not include any form of electrical and/oroptical conductor, and as such is used essentially only to carry axialloading. Such rope may be made from fiber, steel or other high strengthmetal, or combinations thereof. In typical multiple-streamer acquisitionsystems, such as shown in FIG. 1, the laterally outermost towingelements, the paravane tow ropes 16 are in fact typically ropes asdefined above. However, such configuration is not meant to limit thescope of this invention. Other embodiments within the scope of thisinvention may provide cables at the laterally outermost towing positionscoupled between the paravanes 14 and the seismic vessel 10 as shown inFIG. 1. Therefore, as used herein, the tow rope 16 may also be cables.

The streamers 20 are each coupled, at the axial ends thereof nearest thevessel 10 (forward ends), to a respective lead-in cable termination 20A.The lead-in cable terminations 20A are coupled to or are associated withthe spreader ropes or cables 24 so as to fix the lateral positions ofthe streamers 20 with respect to each other and with respect to thevessel 10. Electrical and/or optical connection between the appropriatecomponents in the recording system 12 and, ultimately, the sensors(and/or other circuitry) in the ones of the streamers 20 inward of thelateral edges of the system may be made using inner lead-in cables 18,each of which terminates in a respective lead-in cable termination 20A.A lead-in termination 20A is disposed at the vessel end of each streamer20. Corresponding electrical and/or optical connection between theappropriate components of the recording unit 12 and the sensors 22 inthe streamers 20 may be made, through respective lead-in terminations20A, using lead-in cables 18. Each of the lead-in cables may be deployedby a respective winch 19 or similar spooling device such that thedeployed length of each cable 18 can be changed.

As will be readily appreciated by those skilled in the art, the actuallateral separation between the streamers 20 is related to the tension onthe spreader ropes or cables 24. In one aspect of the invention, theparavanes 14, in cooperation with an adjustable device associated with aparavane bridle (explained in more detail below with reference to FIGS.3 through 9) can be “steered” to exert adjustable lateral force, suchthat the tension on the spreader cables 24 can be controlled, and/or thelateral position of the acquisition system components can be maintainedwith respect to the seismic vessel.

FIG. 2 shows a cross section of one embodiment of a paravane. Theparavane 14 includes several principal components, including a generallylongitudinally extending float or buoy 40 that maintains the paravane 14in a selected position with respect to the water surface and buoyantlysupports the remainder of the components of the paravane 14. The float40 can be coupled, for example, by clamps, brackets or bands 42 to anupper diverter frame 52A. The upper diverter frame 52A provides mountingand support for the uppermost ends of a plurality of substantiallyvertically extending diverters 44, each of which has a selected shapeand orientation with respect to the longitudinal axis of the paravane 14to redirect movement of water as the paravane 14 is towed by the vessel(10 in FIG. 1). Such redirection of the water movement results inlateral force being generated by the paravane 14. In the presentembodiment, the diverters 44 can be supported approximately in theirlongitudinal center by a center diverter frame 52B, and at their lowerlongitudinal ends by a lower diverter frame 52C. Collectively, theframes 52A, 52B, 52C maintain the position of and the orientation of thediverters 44 with respect to the float 40. The diverters 44 are rigidlymounted in the frames 52A, 52B, 52C such that the amount of lateralforce generated depends essentially entirely on the speed of theparavane 14 through the water.

Each diverter frame 52A, 52B, 52C may include respective forward bridlecable couplings, such couplings shown at 56A, 56B, 56C, and aft bridlecable couplings, such couplings shown at 54A, 54B, 54C. An arrangementof a bridle and associated cables according to another aspect of theinvention will be explained below with reference to FIG. 3.

In the present embodiment, electrical power to operate variouselectronic components in a directional controller unit 48 may besupplied by a turbine-powered generator, shown generally at 50 andaffixed to the lower frame 52C. The generator 50 converts flow of thewater past the paravane 14 into rotational energy to drive an electricalternator or generator (not shown separately) disposed therein andcollectively referred to as a “generator.” The exact structure andlocation on the paravane 14 chosen for the generator 50 and for thecontroller unit 48 are matters of discretion for the designer and arenot intended to limit the scope of the invention. The controller unit 48will be explained in more detail with reference to FIG. 10.

As will be appreciated by those skilled in the art, part of the lateralforce produced by the paravane 14 as it moves through the water, as wellas the towing force supplied by the vessel (10 in FIG. 1) are generallycoupled to the vessel (10 in FIG. 1) through the paravane tow rope (16in FIG. 1). Typically, such forces are distributed over a substantialportion or all of the cross-sectional area of the paravane structure,rather than being concentrated at a single coupling point, by couplingthe tow rope (16 in FIG. 1) to the paravane 14 using a device called abridle. The bridle, one embodiment of which will be further explainedbelow, typically includes a plurality of cables or ropes that eachterminate at one end in one of the bridle cable couplings 54A, 54B, 54Cand 56A, 56B, 56C, and each terminate at the other end in one or morebridle “nodes.”

For purposes of describing the forces on the paravane 14 and on thebridle cables, each bridle node acts as a single point. In the inventionthere are two such bridle nodes in each bridle, a forward node and anaft node. Each such node itself can be functionally coupled to the towrope (16 in FIG. 1) by a cable or chain. Referring to FIG. 3, oneexample of an adjustable system that may be used with the paravane ofFIG. 2 will now be explained. As explained above, bridle cables, shownat 54 for the aft bridle cables and 56 for the forward bridle cablesrespectively, each couple at one end to a respective coupling (54A, 54B,54C and 56A, 56B, 56C in FIG. 2) on the paravane diverter frames (52A,52B, 52C in FIG. 2). Also as explained above, the bridle cables 54 and56 are coupled at their other ends to a respective bridle node, shown at54D for the aft node and at 56D for the forward node. The nodes 54D, 56Dare each coupled to one end of a node lead-in line 58 which may be acable or chain. The particular structure used for the lead-in line 58,and the particular device used to control the length of the respectiveportions of the lead-in line 58 are a matter of discretion for thesystem designer. For purposes of defining the scope of this aspect ofthe invention, it is only necessary that the length of lead-in line 58between the coupling point to the tow rope 16 (referred to forconvenience as the “tow point”) and to each of the forward node 56D andthe aft node 54D be controllable so as to be able to control theeffective position of the tow point between the end of the tow rope 16and the nodes 54D, 56D. It should also be understood that the nodes 56D,54D are used to couple cables to distribute forces across the entireforward and aft end, respectively, of the diver frames. In principle,the invention operates by selectively moving the effective tow pointbetween the forward and aft ends of the paravane.

The effective tow point of the tow rope 16 in the present embodiment canbe moved with respect to the paravane in the present using a nodeposition controller 60. The node position controller 60 in thisembodiment provides that the node lead-in line 58 can be coupled at eachof its ends to one of the nodes 54D, 56D, thus effectively coupling eachend of the node lead-in line 58 to a respective end of the paravane. Thenode lead-in line 58 can also be wrapped around a sheave or sprocket 62rotatably mounted within a frame 60A. A sheave is typically used whenthe node lead-in line 58 is in the form of a rope, wire rope or a cable.A sprocket is typically used when the node lead-in line 58 is in theform of a chain. The frame 60A is functionally coupled to the distal endof the tow rope 16 and transmits towing force from the tow rope 16 tothe sheave or sprocket 62, which then transmits towing force to each endof the node lead-in line 58, and thus ultimately to each node 54D, 56Dand to the respective ends of the paravane. In the present embodiment,the distribution of tow forces on each set of the forward bridle cables56 and aft bridle cables 54 can be changed by changing the distancebetween each node 54D, 56D and the frame 60A. Such distance change inthe present embodiment is effected by rotation of the sheave or sprocket62, thus changing relative length of each segment of the node lead-inline 58.

Rotation of the sheave or sprocket 62 in the example shown in FIG. 3,may be effected by a motor/worm gear combination 66 rotationally coupledto a spur gear 64. The spur gear 64 may be rotationally fixed to thesheave or sprocket 62, such that rotation of the spur gear 64 causescorresponding rotation of the sheave or sprocket 62. The motor in themotor/worm gear combination 66 may be an electric motor, pneumatic motoror hydraulic motor. Although other arrangements of motor may be used torotate the sheave or sprocket 62, a motor/worm gear combination ispreferred because such combination will substantially prevent tension onthe node lead-in line 58 from affecting the rotational position of thesheave or sprocket 62, thus maintaining the lengths of the forward andaft segments of the lead-in line 58 fixed. Operation of the motor in themotor/worm gear combination 66 may be performed by selected circuits(not shown) in a control unit (48 in FIG. 2 and FIG. 10), as will beexplained in more detail below. Another example that uses one or moreservo motors coupled to a sprocket using a planetary gear unit will befurther explained with reference to FIG. 7 and FIGS. 8A through 8D.Other devices for rotating the sheave or sprocket 62 may include, forexample, suitably linked hydraulic or pneumatic cylinders.

An example arrangement of the bridle cables 54, 56 when coupled to theirrespective couplings on the diverter frames (52A, 52B, 52C in FIG. 2)and node position controller 60 is shown schematically in FIG. 5.Operating the node position controller 60 to change the relative lengthof the node lead-in line 58 between the tow point and each node 54D, 56Dwill change the “angle of attack” of the paravane (14 in FIG. 1) as itmoves through the water by changing the effective tow point of theparavane (14 in FIG. 1) with respect to the end of the tow rope (16 inFIG. 1).

One particular example of a node position controller 60 is shown inoblique view in FIG. 4. The node position controller 60 in the presentexample may use a chain 158 as the node lead-in line (58 in FIG. 3) andsuch chain 158 may be moved relative to the node position controller 60by rotating a sprocket 90 that is rotatably affixed to the frame (60A inFIG. 3). Rotating the sprocket 90 will cause the length of the chain 158disposed between the tow rope 16 and each node 54D, 56D to changecorrespondingly. The example node position controller, nodes and theirfunctional components will be explained in more detail below withreference to FIGS. 6A through 9.

FIGS. 6A and 6B show opposed oblique views of one of the nodes 56D. Thenode 56D may be assembled from two spaced apart plates 74. The plates 74may include features (not shown separately) configured to retain thebridle cables (54 and 56 in FIG. 3) such as cable retainer pins 72. Thecable retainer pins 72 may each include a larger diameter portion (notshown separately) disposed between the plates 74 and smaller diameterportions on the longitudinal ends of the cable retainer pins 72 that maypass through corresponding openings (not shown separately) in the plates74. Thus when the plates 74 are coupled together, such as by cap screws76A (FIG. 6A) and nuts 76B (FIG. 6B), the cable retainer pins 72 will beretained in between the plates 74. The cable retainer pins 72 thus actas spacers to keep the plates 74 at a selected lateral distance fromeach other. A chain end link 70 may also be retained between the plates74. The chain end link 70 may include through passages (not shownseparately) for the cap screws 76A (FIG. 6A) such that the chain endlink 70 may be held in place by the cap screws 76A as well as forming aspacer to keep the plates 74 laterally spaced apart by a selectedamount. The bridle cables (56 in FIG. 3) may be retained on the cableretainer pins 72 by forming a spliced eye in the end of each bridlecable (56 in FIG. 3) in a manner well known in the art.

One example of a motor that may be used in the present implementation isshown in FIG. 7. The motor 166 may be a direct current (“DC”), brushlessservo motor with integral or separate motor controller. The output shaftof the motor 166 may include a spur gear 166A pressed or otherwiseaffixed directly thereon. The spur gear 166A can be configured to engagea planetary gear unit that will be explained below in more detail withreference to FIGS. 8A through 8C. Rotation of the motor 166 is therebygear-coupled to the sprocket (90 in FIG. 4). The exact type of motorused in any example may be a matter or discretion for the systemdesigner and therefore the type of motor is not a limit on the scope ofthe present invention. As a practical matter it is only necessary toprovide rotational power selectively in each direction to the sprocketin order for the node position controller to work. Other examples mayinclude induction motors, hydraulic motors or pneumatic motors.

The planetary gear unit may be a multiple stage unit to provide veryhigh output torque to drive the sprocket (90 in FIG. 4) using only arelatively low torque electric motor or motors. An output stage of theplanetary gear unit is shown in oblique view in FIG. 8A. A ring gear 80having internal teeth surrounds and makes tooth contact with threeplanet gears 82. The planet gears 82 surround and are in tooth contactwith a centrally positioned sun gear 84. The sun gear 84 is the drivingelement of the output stage of the gear unit, and it is itself driven bythe output of a first stage of the gear unit. The first stage of theplanetary gear unit is shown on top of the output stage in FIG. 8B. InFIG. 8B, the sun gear (84 in FIG. 8A) is obscured from view because itis coaxially coupled below the plane of a driven gear 86 in the inputstage. The driven gear 86 may be rotated by power from three motors(such as the one shown at 166 in FIG. 7) arranged such that their outputshafts, each with a spur gear 166A thereon are in geared contact withthe driven gear 86. Thus, rotation of the motors (see 166 in FIG. 9) iscoupled through spur gears 166A to the driven gear 86. The driven gear86 directly rotates the sun gear (84 in FIG. 8A. The sun gear (84 inFIG. 8A) rotates the planet gears 82, which ultimately rotate the ringgear 80 at a much lower speed and much higher torque than speed andtorque applied by the motors (166 in FIG. 7).

Referring to FIG. 8C, the planet gears (82 in FIG. 8A) may be maintainedin their relative circumferentially spaced positions around the sun gear(84 in FIG. 8A) and inside the ring gear 80 by using a planet carrier88. The planet gears (82 in FIG. 8A) are each constrained to rotateabout a spindle or axle 88A coupled to or formed into the body of theplanet carrier 88. In FIG. 8D, the sprocket 90 may be affixed to theexterior of the ring gear 80 and thus be caused to rotate with the ringgear 80 when the motors (166 in FIG. 9) are operated.

A more detailed view of the node position controller 60 showing onepossible position for the motors 166 is shown in FIG. 9. The motors 166may be affixed to an upper housing 100. The planet carrier (88 in FIG.8C) may also be affixed to the upper housing 100. The planetary gearunit and sprocket 90 may be disposed between the upper housing 100 and alower housing 102 disposed on opposed sides of a controller frame 104.One side of the controller frame 104 may include an attachment loop 116or similar coupling to engage and retain the end of the tow rope (16 inFIG. 3).

One example of a directional controller unit 48 and its functionalinteraction with the recording system (12 in FIG. 1) will be explainedwith reference to FIG. 10. The controller unit 48 may include a radiofrequency transceiver 120 configured to communicate signals between thecontroller unit 48 and a compatible transceiver 112 on or in therecording system 12. The transceiver if functionally coupled to acentral processor (“CPU”) 128 which may be any type of microcontrolleror programmable logic controller (“PLC”) known in the art. The CPU 128is functionally coupled to a motor driver 126, which converts controlsignals from the CPU 128 into electric current to operate the motors(166 in FIG. 9). The CPU 128 may also be in functional communicationwith a global positioning system (GPS) receiver 130. Electrical power tooperate some or all of the foregoing may be supplied by the generator(50 in FIG. 2), the output of which is coupled to a power conditioner122. The unit 48 may include a battery 124 or other energy storagedevice to keep the control unit components energized during times whenthe generator (50 in FIG. 2) is inoperative. In the present embodiment,command signals to operate the node position control unit (54D in FIG.3) may be generated by the recording system in response to, for example,measurements of position made by the GPS receiver 130 indicative of theparavane (14 in FIG. 1) not being in a selected position relative to theseismic vessel (10 in FIG. 1). Using such operating techniques, aselected geometry of the acquisition system may be maintained.Maintaining or controlling other parameters of the acquisition system,for example, maintaining selected tension on the spreader cables or onthe tow ropes, can also be performed using a steerable paravaneaccording to the various aspects of the invention.

A marine acquisition system using steerable paravanes according to thevarious aspects of the invention may be better able to maintainacquisition system geometry notwithstanding making turns in the water,currents in the water and other conditions that would affect thegeometry of the acquisition system.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

1. A paravane for a seismic acquisition system, comprising: a float; aframe suspended from the float; a plurality of deflectors affixed to theframe; a bridle coupled to the frame at selected positions, the bridleincluding at least one forward cable coupled at a first end proximate aforward end of the frame and at least one aft cable coupled at a firstend proximate an aft end of the frame; and means coupled to a second endof each of the forward and aft cables for selectively changing aneffective tow point disposed between the second ends of the forward andaft cables.
 2. The paravane of claim 1 wherein the means for selectivelychanging comprises: a chain coupled at each end thereof to one of thesecond ends of the forward and aft cables; a sprocket rotatably affixedto a housing and in operative contact with the chain, the housingincluding a coupling configured to be operatively affixed to an end of atow rope; a motor affixed to the frame and rotationally coupled to thesprocket; and a controller operatively coupled to the motor, thecontroller configured to operate the motor to rotate a selected amountin response to a control signal.
 3. The paravane of claim 2 wherein themotor comprises a direct current, brushless servomotor.
 4. The paravaneof claim 2 further comprising a reducing gear unit functionally disposedbetween an output shaft of the motor and the sprocket.
 5. The paravaneof claim 4 wherein the reducing gear unit comprises a planetary gearunit.
 6. The paravane of claim 5 further comprising a plurality ofmotors operable to drive an input to the planetary gear unit.
 7. Theparavane of claim 2 further comprising a radio transceiver in operativecommunication with the controller, and wherein the controller isconfigured to operate the motor in response to a control signaltransmitted by a recording unit on a seismic vessel.
 8. The paravane ofclaim 2 further comprising an electric generator associated with theparavane and configured to be driven by movement of the paravane throughwater, the generator configured to supply operating power to thecontroller and the motor.
 9. A marine seismic survey system, comprising:a seismic vessel; a plurality of seismic sensor streamers towed by thevessel at laterally spaced apart positions; a spreader cable extendingsubstantially transversely to a direction of motion of the seismicvessel, each of the streamers coupled at its forward end to the spreadercable; a paravane coupled to each end of the spreader cable, eachparavane including a frame suspended from a float, a plurality ofdeflectors affixed to the frame; a bridle coupled to the frame atselected positions, the bridle including at least one forward cablecoupled at a first end proximate a forward end of the frame and at leastone aft cable coupled at a first end proximate an aft end of the frameand means coupled to a second end of each of the forward and aft cablesfor selectively changing an effective tow point between the second endsof the forward and aft cables; and a tow rope coupled to the tow pointof each bridle at one end and at the other end thereof to the seismicvessel.
 10. The system of claim 9 wherein the means for selectivelychanging comprises: a chain coupled at each end thereof to one of thesecond ends of the forward and aft cable; a sprocket rotatably affixedto a housing and in operative contact with the chain, the housingincluding a coupling configured to be operatively affixed to an end of atow rope; a motor affixed to the frame and rotationally coupled to thesprocket; and a controller operatively coupled to the motor, thecontroller configured to operate the motor to rotate a selected amountin response to a control signal.
 11. The system of claim 10 wherein themotor comprises a direct current, brushless servomotor.
 12. The systemof claim 11 further comprising a reducing gear unit functionallydisposed between an output shaft of the motor and the sprocket.
 13. Thesystem of claim 12 wherein the reducing gear unit comprises a planetarygear unit.
 14. The system of claim 13 further comprising a plurality ofmotors operable to drive an input to the planetary gear unit.
 15. Thesystem of claim 10 further comprising a radio transceiver in operativecommunication with the controller, and wherein the controller isconfigured to operate the motor in response to a control signaltransmitted by a recording unit on a seismic vessel.
 16. The system ofclaim 10 further comprising an electric generator associated with eachparavane and configured to be driven by movement of the paravane throughwater, each generator configured to supply operating power to thecontroller and the motor.
 17. A paravane for a seismic acquisitionsystem, comprising: a float; a frame suspended from the float; aplurality of deflectors affixed to the frame; means for coupling a towrope to a lead-in functionally extending between a forward end and anaft end of the frame; and means for selectively changing an effectiveposition along the lead-in of the means for coupling the tow cable. 18.The paravane of claim 17 wherein the means for selectively changingcomprises: at least one of a chain and a cable coupled at each endthereof to one of a forward bridle node and an aft bridle node, thenodes each including at least one cable coupled to a respective end ofthe frame; at least one of a sheave and a sprocket rotatably affixed toa housing and in operative contact with the chain, the housing includinga coupling configured to be operatively affixed to an end of a tow rope;a motor affixed to the frame and rotationally coupled to the at leastone of a sheave and a sprocket; and a controller operatively coupled tothe motor, the controller configured to operate the motor to rotate aselected amount in response to a control signal.
 19. The paravane ofclaim 18 wherein the motor comprises a direct current, brushlessservomotor.
 20. The paravane of claim 19 further comprising a reducinggear unit functionally disposed between an output shaft of the motor andthe at least one of a sheave and a sprocket.
 21. The paravane of claim20 wherein the reducing gear unit comprises a planetary gear unit. 22.The paravane of claim 21 further comprising a plurality of motorsoperable to drive an input to the planetary gear unit.
 23. The paravaneof claim 18 further comprising a radio transceiver in operativecommunication with the controller, and wherein the controller isconfigured to operate the motor in response to a control signaltransmitted by a recording unit on a seismic vessel.
 24. The paravane ofclaim 18 further comprising an electric generator associated with theparavane and configured to be driven by movement of the paravane throughwater, the generator configured to supply operating power to thecontroller and the motor.